Pleated sac for patient support mattress

ABSTRACT

A support sac for a patient support mattress comprises an elongate sac formed of a pressure retaining flexible material adapted to be fluid filled under pressure. The sac has a body contact portion (10) remote from a support surface portion (22), the body contact portion being provided with a plurality of pleats or ruffles whereby said contact portion has a surface area per unit length significantly greater than the surface area per unit length of the support surface portion thereby to alleviate hammocking effects.

The present invention relates to sacs for patient support mattresses ofthe static pressure, alternating pressure and low air loss type.

Alternating pressure type air beds are to be found described inGB-A-1595417. Briefly this document describes an air bed for patientsupport, for example in a burns unit or in geriatric care, whichcomprises a plurality of overlying flexible pressure resistant tubeswhich are inflated and deflated in an alternating cycle over about 5 to10 minutes. The purpose of this is to reduce the period of time that animmobile body tissue carries weight which could shut off or reduce bloodflow. Thus every five minutes or so every skin tissue is unstressed byweight and hence where blood flow is weak the capillary system canoperate to oxygenate the unstressed tissue.

The effect of this is for example to greatly assist the prevention andhealing of bed (pressure) sores. There are a number of differentdevelopments of this type in use, all of which have as a goal thereduction of point loading of particular parts of the body on analternating basis. As will be apparent bony prominences such as thesacrum are particularly prone to cause problems with tissue perfusion insemi-immobile geriatric patients, or those who have undergonesignificant surgery. One example of an air bed of this type is in themarketplace under the trade mark "Pegasus".

Another approach to this problem has been the use of the low air lossbed concept, for example, that marketed under the trade mark "Mediscus".In this arrangement, the patient is in effect supported on air containedin a plurality of water proof, but water-vapour permeable, air sacs. Airis caused to flow through the air sacs at about 140 m³ /hr. Theplurality of sacs are arranged in, for example, four groups of fivesacs, and each is adjusted to a pressure suitable for support of a partof the patient's body. Again the air sacs used are essentially flexiblealthough they are made of a material which allows for a degree ofresilience.

In another type of fluid filled mattress, a static pressure air filledmattress is provided. The mattress is valved so that the pressure can beadjusted to a desired value when the patient is in situ on the mattress.

All these mattress types suffer to a greater or lesser extent from theproblem of "hammocking". Hammocking is the effect which takes place whenthe weight of a human body is transferred to pressure bearing flexiblecontact faces such as tubes. The effect of point loading on such tubesis to cause local stretching (to an extent) such that a portion at leastof the loading is accepted, not by the pressurised air, but by thestretching of the material of the tube. This causes localised pressuresto rise and induces shear forces between the skin and the material, andhence increases the loading of certain areas of the skin adjacent forexample a bony prominence. These arrangements therefore can be improvedsuch that the weight transfer is nearer to the ideal, in that the wholeweight of the patient's body is borne by a fluid (i.e. the pressureexerted over a section of a patient's body is more equalised whateverthe body shape).

The Applicant has now discovered that the above ideal can be more nearlyapproached by arranging that the body contact surface is provided with apleated or ruffled contact surface, particularly one in which thelongitudinal length of the sac is maintained by a longitudinallyextending substantially non-extensible member. This allows bodyprominences to sink into the surface material with much reducedhammocking thereby more readily transferring the patient's weight to thefluid and so tending to equalize pressure relative to adjacent skinportions.

Accordingly support sacs for use in the foregoing mattress types can beimproved in performance by the arrangement of the present invention. Theinvention also provides that the fluid may be any suitable gas, vapour,liquid or gel or a mixture thereof, so long as it fulfils therequirements of the alleviation of hammocking.

According therefore to the present invention there is provided anelongate support sac for a patient support mattress which comprises asac formed of a pressure retaining flexible material adapted to be fluidfilled under pressure, characterized in the sac comprises a body contactsurface portion remote from a support surface portion, and in that thebody contact surface portion is provided with a plurality of ruffles orpleats. Said body contact portion may have a surface area per unitlength significantly greater than the surface area per unit length ofthe support surface portion thereby alleviating hammocking.

Preferably the sac contains at least one longitudinally extendingsubstantially non-extensible member thereby to reduce or restrain thesac from longitudinal extension to ensure that the pleats and/or rufflesremain in place under load. The non-extensible member may be a string orcord but is more preferably continuous or partially discontinuoustubular member to which the pleats may be secured. It is more preferablethat the non-extensible member when used should be positioned as far aspossible from the contact surface to avoid uncomfortable contact withthe patient.

In one embodiment the ruffles or pleats may extend normal to the axis ofthe sac. These may be in the form of one or more interconnected arcuatemembers so that the cross-section of the sac is provided with at least"ear" portions.

These "ear" portions may be ruffled or pleated in the horizontaldirection or not depending upon the eventual use of the sac. Where thesac is provided with one or more "ear" portions, these may be unitary,i.e. conjoined to each other and not to the central longitudinallyextending portions of the sac or may be formed of at least twooptionally pleated tubes having their axis parallel to that of the sacand operatively interconnected therewith.

In a preferred form of the invention, the sac may be filled with a gas,liquid, gel or low friction powder and is connected to a source ofpressurization. Preferably the sac is adapted to be filled bypressurized air and optionally includes means for alteration of theinternal pressure of the sac on a cyclical basis.

The pleating is conveniently disposed perpendicular to the length of theair sac, but of course, may also be longitudinal of its axial length, orangled to the axis of the sac. The term pleated as used herein includesthe term ruffled.

In low air loss beds, the air sacs of the adjacent group are not subjectto alternating pressure. In such cases, air sacs of the presentinvention may be used. Particularly suited to such an embodiment areeffectively single celled elongate air sacs provided with upper contactand lower support, surfaces, especially those in which the upper contactsurface is in cross-section bifurcated or trifurcated.

While the support surface is generally planar, the upper contact surfaceis pleated. An approximation of the ratio of the lower to the uppersurfaces is 1:1.67 to 1:2.0 in terms of material length before anypleating.

In alternating pressure air beds, because they tend to operate at higherpressure, each air sac of the mattress may be more complex becauseotherwise the air sac tends to adapt a "banana" shape. Accordingly,between the upper contact, and the lower support surface is interposedan intermediate portion which is at least substantially unpleated andacts as the longitudinal non-extensible member. By means of carefuldesign, the tendency of the upper contact surface to deform can becounteracted by an approximately equal and opposite tendency of thelower support surface. This deformation tendency can be adjusted with anaverage patient weight in mind to give a better result.

Accordingly, a preferred feature of the invention provides an air sacformed of a flexible material for operative connection to a source ofpressurised gas; characterised in that the air sac comprises an upperruffled or pleated body contact portion and a lower pleated supportportion, and in that an intermediate portion is interposed between saidupper and lower portions, said intermediate portion being at leastsubstantially unpleated such that its axial longitudinal extension islimited. The pleats preferably extend onto the base portion.

In another feature of the invention, where the non-extensible member ispositioned remote from the contact surface, a plurality of transversesupports may be provided between the non-extensible member and thecontact surface to restrain the size of the sac from excess arcuatedeformation; while also reducing the possibility that the patient canfeel the transfer supports in use via the contact surface. In apreferred embodiment of this type, the pleat/ruffle ratio of the uppercontact portions is between 1:1.57 to 1:2.5 with the higher values beingindicative of the ratio of the skin contact surface.

The support portion may be similarly pleated along its length, but in apreferred embodiment is pleated only over a central section of about 1/2to 1/3 the axial length. The pleat/ruffle ratio of the low supportportion is 1:2.0 to 1:3.0, preferably about 1:2.5 with the higher valuesbeing associated with the support surface such as a bed base or a flator shallow V-shaped mattress.

The air sac of the present invention may be used substantially as astack which extends along the axis of, but perpendicular to, a mattressor bed base. Each air sac may be connected to a source of pressurisedgas (e.g. air) so that the pressure in the air sacs can be adjustedeither to a constant or alternating value as required by the intendedfinal use. Each sac is preferably provided with areas for retaining thesame in position, for example, a reinforced ring through which asecuring cord may pass.

The air sacs may be fabricated from any flexible and/or resilientmaterial, for example, polyurethane or PVC or a fabric supportedpolyurethane. The pleats may be stuck and/or sewn or vacuum formed andthen welded as appropriate. The material may be arranged to seep air.

In an alternative method of manufacture, the pleats or ruffles may beformed by causing relative expansion of the body contact portion orrelative reduction in axial length of the intermediate portion when theintermediate portion is used.

In the case of a single sac, the sac may be formed as a preform andinserted into a heated mould. The preformed sac is filled with air or aninert gas and heat is then applied differentially such that the bodycontact portion is heated to a significantly higher value than thesupport portion. With the positive air pressure within the formed sacbut with the support portion retained by the mould, and since thetemperature of the body contact portion is locally raised above thethermoplastic threshold of the material used, the body contact portionwill deform non-elastically to form pleats and/or ruffles.

Alternatively and/or additionally vacuum forming may be used to achievethis end.

Accordingly therefore to a further aspect of the present invention thereis provided a method for the production of a support sac for a patientsupport mattress which comprises:

providing an arcuate moulding member defining an inwardly directedchannel, said channel being provided with means for inducing a negativepressure therewithin;

securing a heat and/or pressure deformable sheet of plastics materialover the free ends of said channel, and applying heat and negativepressure to said mould thereby to vacuum form an arcuate member of agenerally u-shaped configuration in said plastics material,

and subsequently after, or during, cooling straightening the arcuatemember thereby to induce pleats and/or ruffles in said intended contactsurface. The free edges of the arcuate member may then be secured to alongitudinally non-extensible intermediate member by welding, sticking,sewing etc.

In another method for the manufacture of the pleated/ruffled sac,particularly where an intermediate, non-axially stretchable portion isused, the intermediate portion may be formed of a material which afterfabrication without pleats, for example by moulding and/or welding ofdifferent but compatible plastics materials, is axially shrinkable, forexample by the application of heat. In all these arrangements when usinga part-welded material, the application of heat will also result in animproved bonding for the welds.

Where more than one sac portion is used in a stacked orientation, it isdesirable that the diameter of the uppermost, (i.e. the body contactportion) shall be greater than that of the sac portions beneath. Thus,where there are three sac portions the body contact portion shouldpreferably have a diameter greater than that of the intermediate portionwhich in turn may have a diameter greater than the support portion. Thisis to ensure that since the internal pressure of the sac is a singlevalue, the sac collapses under point load in a structured way so thatthe patient does not tend to feel the harder portions of the sac underhigh point-loads.

The invention will now be described by way of illustration only withreference to the accompanying drawings wherein:

FIG. 1 shows a diagrammatic view of a test air tube of the prior artwith indentor applied, from below;

FIG. 2 shows a graph of interface pressure against indentor pressure atgiven distances from the centre thereof in mm/Hg. The full line showsthe prior art values while the broken line shows the invention at thesame pressure;

FIG. 3A shows a low air loss sac as a polymethane blank with airinput/output attached, while FIG. 3B shows the sac of FIG. 3A in itsfinal pleated form;

FIG. 4A shows a side view of an alternating pressure air sac of a firstform with a high pleat ratio but reduced length support area; whilstFIG. 4B shows a side view of an alternative form of air sac;

FIG. 5 shows from above an air bed in a diagrammatic form retained in abed base;

FIG. 6 shows an arrangement of FIG. 4 in vertical cross-section along aline AA of FIG. 4B;

FIG. 7 shows in diagrammatic side elevation a mould with a plasticsmaterial applied thereto;

FIG. 8 shows a body contact portion obtained from the mould of FIG. 7prior to welding to an intermediate portion;

FIGS. 9A to 9C show a transverse cross-section through various sacs inaccordance with FIG. 3B and FIG. 4A; and

FIGS. 10, 10A and 11 show views of an elongate tube incorporating a sacarrangement of the present invention.

Referring to FIGS. 1 and 2, these show the effects which occur when aninflated cell deforms under load of an indentor. The indentor is arounded object adapted to mimic the effect of a bony prominence. Thedeformation is referred to as "hammocking" which results in the measuredinterface pressures being higher than the nominal air pressure in thecell because of the additional forces from tensioned skin.

The relationship between the measured interface pressure Pi and the airpressure within the cell Pa may be expressed as: ##EQU1## r is theradius of the cell, R is the radius of the indentor.

This may be seen in FIG. 1 wherein the indentor having a radius of 9.5cm is applied to a polyurethane tubular cell of radius 3.5 cm. Thisgives a hammocking ratio of about 1.37. The hammocking ratio is expectedto remain at that factor up to any usable pressure in the cell. It willbe appreciated that where the contact portion of the body, e.g. theheels are of small radius, the value of R should be reduced as well toachieve best results.

With the Indentor centrally located on the tubular inflated cell, itwould be expected that the maximum interface pressure would be foundcentrally. This was not the case however, in that in practice it wasfound that there was a significant drop in interface pressure at thedomed central contract point, with an increased pressure readingdisposed there about. Typical figures are shown in FIG. 1 and separatelyin the full line of FIG. 2. This effect is due in part to materialdisplacement under pressure, and is due in part to stretching of thematerial under load. Where the object imparting the load is the skin,this extra pressure is applied thereto obturating the capillary networkand starving the skin of oxygenated blood.

With reference to FIG. 1, it will be seen that the tubular air sac (1)is inflated to about 60 mm/Hg and then the indentor (2) is appliedthereto. Where the force on the central point of the indentor is ±68mm/Hg, the other relative figures hold good in approximate proportion.This gives rise to the full line graph (3) in FIG. 2.

The broken line graph (4) in FIG. 2 shows the predicted results obtainedfrom a pleated cell of the invention under the same conditions. It willbe noted that the graph is significantly flatter.

The air sacs of the invention are of two types in the present examples.The low air loss type of FIGS. 3A and 3B and the alternating airpressure types of FIGS. 4A and 4B.

With reference to FIG. 3A, two trapezoidal blanks (5) of a fabricreinforced polyurethane material were superimposed and welded alongtheir joint edges; an air-inlet/outlet (6) was also formed therein in anair tight fashion.

The longer edge of the trapezoidal section was either then, orpreviously had been, formed with pleats (7) shape as shown in FIG. 3B.This may be done by adhesively securing and/or stitching, and thenwelding or a combination of all or some of these. As a result, the bodycontact portion at the top of the section has a surface area per unitlength greater than the surface area per unit length of the effectivesupport surface portion at the bottom of the section.

The air sac so formed may be positioned on an air bed as shown in FIG. 5to be described later.

Alternatively the arrangement of FIG. 4A may be used in an alternatingair pressure bed (8). In this arrangement, an elongate tube (9) isformed in operative interconnection with an upper pleated contactportion (10) and a lower pleated support portion (11).

The upper pleated contact portion (10) has a pleat ratio of about 1.67relative to the unpleated elongate tube (9), the area at the interfaceof the upper pleated contact portion (10) and the elongate tube (9)serving as the effective support surface portion. The lower surfacesupport (11) is pleated but only centrally at (12), and preferably at ahigher ratio than the upper pleated portion (10). Alternatively as shownin FIG. 4B the lower support portion (13) has a pleated configurationover its full length. The purpose of the lower support surface pleats(12) and (13) is to balance the "banana" effect of the upper pleatedparts.

The full depths of the air sacs of FIGS. 3A to 4B is of the order of 200to 300 mm.

FIG. 6 shows a vertical section through an arrangement of FIG. 4B. Inthis arrangement the body contact portion (10) has a diameter largerthan that of the intermediate elongate tube (9) which again has adiameter larger than the lower support portion (13). In this instance,body contact portion has a circumference (transverse) of about 34 cm,the elongate tube (9) has a circumference of about 28 cm and the lowersupport portion (13) has a circumference of about 20 cm. A plurality ofapertures (16) interconnect the tube (9) with the lower support portion(13) and with the body container portion (10) thereby to equilibrateinternal pressure.

As shown in FIG. 5, a bed base (14) retains a plurality of sacs (8). Inthe arrangement shown, the plurality of sacs (8) are each supplied withair pressure via an input/output port (6), each independently connectedto an air pressure controller/timer (not shown) and via that to a pumpas shown in ghost lines at (15).

With the air sacs inflated to 60 'mm/Hg and by use of the indentor ofFIG. 1, a pressure profile of the broken lines of FIG. 2 may beachieved.

With reference to FIGS. 7 and 8, FIG. 7 shows an arcuate mould (20)having a generally u-shaped transverse cross-section with the free endsof the U-shaped facing inwardly. The mould (20) has a plurality of thevacuum points (21) disposed at regular intervals about the mould andcommunicating with the interior of the channel. By means of these,vacuum may be applied to the interior of the channel member inappropriate circumstances.

A rectangular sheet of a plastics material (22) is disposed over andsecured to the inner edges of the mould so as to form an air-tight seal.Heat (23) shown diagrammatically may then be applied to the material(22) while vacuum is applied via the vacuum portion (21). When theplastics material has been fully formed, heat and pressure is withdrawnand the formed material is allowed to cool. When cooled, or as it cools,the formed plastics material is removed from mould (20) and straightenedso as to adopt a shape shown in FIG. 8 with flanges (22) for welding,completed portions (10) as shown in FIG. 4A and 4B. The flange portion(22) may be welded to the longitudinally non-extensible intermediatemember (9) by traditional means.

As shown in FIG. 6 a series of apertures (16) are disposed between theintermediate portion (9) and the pleated portions (10) and (14) so as toallow pressure equilibration. These ports (16) may be replaced by aplurality of discrete strips to form a ladder like configuration in analternative.

FIG. 9 shows at FIG. 9B a transverse cross-section through a sac of FIG.3B and in FIGS. 9A and 9C, a transverse cross-section through a sac ofFIG. 4.

With reference particularly to FIG. 9B, there is shown a support section(5) integrally formed with upwardly extending double pleat members (10A)to form a single plate (10). The point of conjunction (33) of thedouble-pleat members (10A) is provided with ties (32) to secure the sameagainst vertical expansion. This arrangement is particularly suitablefor low-loss air beds particularly where the completed portion (10) ispleated in the longitudinal direction as shown in FIG. 3B.

FIG. 9A shows a transverse cross-section to the arrangement of FIG. 4A.

In accordance with the arrangement previously described, an elongatetube (9) is formed in operative connection with an upper (4) pleatedcontact portion (10) and a lower pleated support portion (11). These arewelded together by longitudinal welds (22) in an air-tight fashion. Theupper pleated portion (10) in provided with a bifurcated portion (10A)which is pleated in the longitudinal direction as shown in FIG. 4A, andin the transverse direction as shown in FIG. 9A. The bifurcated pleatedportion (10A) are conjoined at (10B), which conjuncture (10A) may belinked to the material of the sac by means of cords to prevent upwardunfurling of the bifurcated portion. The effect of this is to increasethe surface area available for prevention of hammocking.

FIG. 9C relates to a basic type of air sac. A longitudinally extendingsupport tube (30) is provided at its upper contact surface with a pairof longitudinally extending tubes (31), the contact surfaces of whichmay be pleated or unpleated. It will be appreciated that the internalpressure of the contact tube (30) is the same as that of each tube (31).It will also be appreciated that in the unpleated condition, the contacttube (31) provide a more hammocking resistant surface than would asingle tube, and similarly that in a pleated condition the arrangementprovides a simpler but effective unit for resisting hammockingparticularly when used in conjunction with a foam layer.

FIGS. 10, 10A and 11 show a further sac arrangement of the invention. Asshown, an elongate tube (40) includes an upper support portion (41) anda lower support portion (43), separated by a transverse internal supportmember (42). The support member (42) is provided with apertures to allowfluid to flow freely between portions (41) and (43).

The tube (40) is formed by welding together two "banana" shapedstructures corresponding to the upper and lower support portions (41)and (43) as illustrated in FIG. 11.

The portions (41) and (43) are welded together at cross-hatched areas"W" of each banana shape. In practice, only a single transverse supportmember (42) is provided at the area of contact, the member (42) servingas the effective support surface portion. The excess material in theside faces (44) of the banana-shaped structures corresponding toportions (41) and (42) forms pleats in the tube (40) when the tube isstraightened as shown in FIG. 10. Such pleats act to prevent hammocking.

EXAMPLE 1

In order to compare the pressure signatures achievable by standardcommercial air beds, an air bed substantially as shown in FIG. 5 andincorporating air bags as shown in FIG. 4A (the invention) was testedagainst a standard foam NHS mattress, a "Pegasus" Air Mattress and the"Nimbus 2" Air Mattress.

This was effected by positioning a pressure pad at the sacrum, theischial protuberance, the heel and the trochanter of a series of healthyvolunteers and measuring maximum and minimum pressure values in eachcase and averaging the same. The minus percentage values arc thereforeparticularly desirable since they indicate the percentage improvementover standard values.

In this example, the "Oxford Pressure Monitor Mark II" was used for allreadings. This determines the interface pressure for observation of thepressure flow characteristics of small pulses of air.

The positioning of the sensors were as follows:

Semi-recumbent--right heel, left heel, sacrum, left ischial tuberosity,right ischial tuberosity.

Side Lying--left trochanter.

The heels and the ischial/sacral region are chosen for investigation asthey represent a comparatively light yet bony area in the case of theheel whereas the sacrum/ischial tuberosities when semi-recumbent have anarea which has considerable loading over a much larger surface.Therefore in making measurements at these sites, the performance of themattress to a variety of loading patents can be assessed. The trochanteris chosen and is often considered one of the most difficult areas inwhich to achieve satisfactory pressure relief.

The subject used in this evaluation were all members of SalisburyHospital Retirement Fellowship and hence have an average age of over 70years. All are ambulant and in good health. None have any current soresor a history of pressure sores. During the tests they all wear standardnight wear or loose fitting underclothes. A single sheet is placed overeach mattress. It is not tucked in but left to lie loose, all wrinklesbeing smoothed out.

For the measurements on the sacrum, ischials and heels, the back rest isleft at 45° and the mattress is placed over the top of the back rest.Two pillows are used to support the head. For the measurement on thetrochanter, the beds are flat with the subjects' heads being supportedon two pillows. Both hip and knee angles are at 60° by using speciallyconstructed foam templates.

The 3*4 sensor arrays are taped to the skin over the sacrum and overboth ischials with the subject standing with the hips flexed at 45° in asimilar position to that which they will adopt on the bed during thetests. Care is taken to ensure that there is no tape across the surfaceof any of the pressure elements. The subject is then carefullytransferred onto the bed and the bed is checked to ensure none of thesensors are creased or folded.

If any problem is found, the sensor is then inspected and any creasesremoved and the pressure is remeasured. The subject then remainsstationary on the bed for two complete cycles so the bed can adjust tothe weight of the subject. The readings are than taken in two completecycles for the sacrum and one complete cycle for the ischialtuberosities. The maximum, minimum and average pressures measured overone or two cycles is then printed out using a programming facility. Footblocks are adjusted to ensure that the patient does not slide down thebed during tests.

Pressures are measured on the heels using individual cells connected toa standardized computer. One sensor is taped to each heel over the pointof maximum pressure and the heel is positioned on the ridge of themattress with the foot hold vertically. The block or board used toprevent the subject from slipping down the bed in the sacral readings isnot used as false readings can occur due to some pressure being taken onthe soles of the feet.

However, the block or board is used as an indicator to observe if thesubject slips down the bed. Readings on both heels simultaneously on twocomplete cycles. Again the maximum, minimum and average pressures arerecorded. The head is supported on two pillows and the subject is leftin the position of two complete cycles during which time measurementsare taken to ensure the sensor is lying correctly before the actualreadings are taken again over two cycles. Maximum, minimum and averagepressure readings are then recorded.

The ten volunteers who participated in the study had an average age of67 years and an average weight of 60.9 kg and an average height of 1.64m. For each given subject, the maximum pressure measured on eachlocation on each mattress was noted. This maximum pressure and thecorresponding minimum and average pressures for the same sensor werethen used to calculate a mean average pressures and examined deviationsfor all subjects on that mattress. The results obtained in that way wereshown in Table I.

                  TABLE I                                                         ______________________________________                                                STANDARD                                                                      NHS                                                                   CONTRACT                                                                              AVERAGE    MAXIMUM   MINIMUM                                          AREA    MATTRESS   PRESSURE  PRESSURE                                                                              PRESSURE                                 ______________________________________                                        SACRUW  74         39        61      27                                       Mean    17         18        16      13                                       St. Dev.           -48%      -18%    -64%                                     Conf. Int.                                                                    % Pressure                                                                    ISCHIALS                                                                              54         41        61      25                                       Mean    16         13        14      11                                       St. Dev.           -24%      +13%    -54%                                     Conf. Int.                                                                    % Pressure                                                                    TROCH.  99         43        65      20                                       Mean    24         10        18      8                                        St. Dev.           -57%      -34%    -80%                                     Conf. Int.                                                                    % Pressure                                                                    HEELS,  112        74        115     24                                       FULL CELL                                                                             23         14        21      9                                        Mean               -34%      +3%     -79%                                     St. Dev                                                                       Conf. Int.                                                                    % Pressure                                                                    HEELS,  113        29        58      10                                       HALF CELL                                                                             15         29        37      21                                       Mean               -74%      -49%    -91%                                     St. Dev.                                                                      Conf. Int.                                                                    % Pressure                                                                    AVERAGE            -41%      -9%                                              PERCENT                                                                       PRESSURE                                                                      FULL CELL                                                                     AVERAGE            -22%      -72%                                             PERCENT                                                                       HALF CELL                                                                     ______________________________________                                    

The results presented above have been obtained using the protocoldescribed above and developed from the Department of Health Mattressevaluation project.

As such, these readings are directly comparable with those published inthe Department of Health Publication PS4 to be published shortly. It canbe seen that the results of the product are impressive especially afterthe large cells on the heals were replaced with cells of half of thewidth. The initial reading on the first two volunteers showed that somedesign modifications were needed to improve the performance of themattress in the sacral/pelvic area when the subject was in asemi-recumbent position. Hence the readings presented here on the sacrumand ischials are only on eight subjects not on ten.

There are six sets of readings on the large heel cells and four sets ofreadings on the half sized heel cells which are shown to make asubstantial improvement. As both heels are measured the statisticalanalysis was undertaken with twelve and eight readings of full and halfheels respectively.

The readings on this product compare very well with market leaders whichare measured as part of the Department of Health Trial. These were shownin Table II below.

                  TABLE II                                                        ______________________________________                                                       PEGASUS      NIMBUS                                                           Product A    Product B                                         ______________________________________                                         Sacrum                                                                       Ave            51 mm/Hg     41 mm/Hg                                          Max            88 mm/Hg     85 mm/Hg                                          Min            23 mm/Hg     28 mm/Hg                                          Ischials                                                                      Ave            42 mm/Hg     34 mm/Hg                                          Max            67 mm/Hg     52 mm/Hg                                          Min            25 mm/Hg     26 mm/Hg                                          Trochanter                                                                    Ave            49 mm/Hg     49 mm/Hg                                          Max            92 mm/Hg     78 mm/Hg                                          Min            22 mm/Hg     34 mm/Hg                                          Heels                                                                         Ave            68 mm/Hg     77 mm/Hg                                          Max            174 mm/Hg    139 mm/Hg                                         Min            12 mm/Hg     118 mm/Hg                                         ______________________________________                                    

The average pressure reduction figures over all sites on the two marketleaders were then compared with the inventive system below:

                  TABLE III                                                       ______________________________________                                        1st Tech (large                                                                           1st Tech (small                                                   heel cells) heel cells)                                                                              Pegasus A                                                                              Nimbus B                                      ______________________________________                                        Ave      -41%   -51%       -30%   -35%                                        Max      -9%    -22%       +30%   +10%                                        Min      -69%   -72%       -69%   -62%                                        ______________________________________                                    

Negative pressures correspond to a reduction in pressure and positivevalues and relate to an increase in pressure compared to the standardhospital mattress values of which are given in Table I.

As can be seen, the air mattress in accordance with the presentinvention provides improved comparative pressure signatures throughoutthe range of pressures points. This is particularly so with reference tothe heal area shown as "D" in FIG. 2 which shows a higher average valuefor the heel area.

It will be understood that the embodiments illustrated show applicationsof the invention in certain forms only for the purposes of illustration.In practice, the invention may be applied to many differentconfigurations, the detailed embodiments being straight forward forthose skilled in the art to implement.

For example, foregoing arrangements have been described with referenceto inflatable air tubes or sacs, but it will be appreciated that staticfluid filled sacs equally benefit from the alleviation of the sores asdescribed.

The invention provides therefore a novel patient support mattress.

I claim:
 1. A support sac for a patient support mattress which comprisesan elongate sac formed of a pressure retaining flexible material adaptedto be fluid filled under pressure, characterized in that the saccomprises a body contact portion remote from a support surface portion,and in that the body contact portion is provided with a plurality ofpleats, or ruffles whereby said body contact portion has a surface areaper unit length significantly greater than a surface area per unitlength of the support surface portion, thereby to alleviate hammockingeffects.
 2. The sac according to claim 1 wherein the sac is filled witha fluid selected from a gas, a liquid, a gel or a low friction powder.3. The sac according to claim 1 wherein the sac is adapted forconnection to a source of fluid pressure.
 4. The sac according to claim3 wherein the sac is air filled and connected to a pump for alterationof the internal pressure thereof.
 5. The sac according to claim 1wherein the pleats extend generally perpendicular to the axis of thesac.
 6. The sac according to claim 5 wherein the sac is effectivelysingle celled, and wherein the body contact portion is generallypleated, and the support surface is generally unpleated.
 7. The sacaccording to claim 6 wherein a pleat/ruffle ratio of the body contactportion is 1:1.67 to 1:2.0.
 8. The sac according to claim 5 wherein thesac comprises an intermediate portion interposed between said bodycontact portion and said support surface portion, said intermediateportion being substantially unpleated and limiting longitudinalextension of the sac, said support surface portion disposed lower thanthe body contact portion and having a plurality of pleats.
 9. The sacaccording to claim 8 wherein the support surface portion is pleated overa central portion thereof, the pleat/ruffle ratio of said centralportion is higher than the pleat/ruffle ratio of the body contactportion.
 10. The sac according to claim 9 wherein the support surfaceportion is pleated only over the central portion.
 11. The sac accordingto claim 8 wherein the diameter of the pleated body contact portion islarger than the diameter of the support portion.
 12. The sac accordingto claim 11 wherein the support surface portion and the intermediateportion have the same diameter.
 13. The sac according to claim 11wherein the support surface portion has a diameter that is less than adiameter of the intermediate portion.
 14. A mattress comprising aplurality of adjacent sacs for a patient support mattress, each one ofthe sacs comprising an elongate sac formed of a pressure retainingflexible material adapted to be fluid filled under pressure, each one ofthe plurality of sacs comprising a body contact portion remote from asupport surface portion, the body contact portion having a plurality ofpleats, said body contact portion having a surface area per unit lengthsignificantly greater than a surface area per unit length of the supportsurface portion whereby hammocking effects are alleviated.